The present invention is directed to weigh-in-motion (WIM) systems for weighing moving vehicles. More particularly, the present invention is directed to a WIM system having the ability to automatically determine and periodically apply calibration factors to WIM scale readings.
Weigh-in-motion (WIM) systems permit the weighing of vehicles, such as tractor trailers, while the vehicles are in motion. Thus, WIM systems can minimize the number of vehicles that must be diverted from a roadway to a static scale or static weighing station.
Various types of WIM scales currently exist. Commonly, but without limitation, WIM scales may be of piezoelectric sensor, quartz digital sensor, bending plate or load cell design. Regardless of the particular design of a WIM scale, however, there are certain difficulties associated with the weighing of moving vehicles. Particularly, the various dynamic forces exerted on a WIM scale by a moving vehicle can effect its weight reading and, therefore, the detected weight of an associated vehicle may be inaccurate. Vehicle characteristics such as classification, length, weight, and speed can all influence the output of a WIM scale. Non-vehicle characteristics such as the condition and levelness of the pavement surrounding a WIM scale can also effect weight readings output thereby.
These difficulties generally result in WIM scales having a degree of accuracy that is less than that of static weighing systems. In order to improve the accuracy of a WIM scale, a calibration factor may be applied thereto. The calibration factor is used to counter the undesirable effects that the aforementioned dynamic loading and/or site conditions may have on the weight readings of a WIM scale.
Currently, such calibration factors must be determined and applied by a manual process. Generally, at least certain of the vehicles crossing a WIM scale are identified and directed to a static scale. The weight of a vehicle as determined by the static scale, is compared to the weight of the vehicle as estimated by the WIM scale. Comparisons between the WIM estimates and the static weights for a number of vehicles are made, and a calibration factor is determined. The calibration factor is then applied to the vehicle weight readings output by the WIM scale to ensure that the weight estimates are as close as possible to the true static weights of the associated vehicles.
Unfortunately, current WIM scale calibration processes are manual in nature. That is, the vehicles whose weights will be compared are manually identified at the WIM scale, and again at the static scale to ensure the two weight readings are associated with the same vehicle. Recording of vehicle weight readings and subsequent calibration factor determination is also a manual process. Further, initial application of a calculated calibration factor and/or updating of a calibration factor are also manual processes. Because such manual processes are commonly time consuming and/or costly, WIM scale re-calibration is typically performed on an annual basis at best, if at all. This is problematic because changes in site and/or WIM scale conditions may demand more frequent calibration factor updates. As a result, WIM systems may provide inaccurate vehicle weight estimates.